Nematic liquid crystal bistable display device with grey level

ABSTRACT

The present invention relates to a nematic bistable display device, characterized in that it comprises control means ( 40 ) for producing, after the anchoring is broken, hybrid textures in which two bistable textures coexist in a controlled proportion within the same pixel, these being separated by 180° volume disclination lines or by 180° reorientation walls on one of the surfaces, and means ( 40 ) for the long-term stabilization of the hybrid textures by transformation of the volume lines into surface walls and the immobilization of these walls on the surface.

The present patent application is a non-provisional application ofInternational Application No. PCT/FR02/01514, filed May 2, 2002.

The present invention relates to the field of display devices based onliquid crystals.

Liquid crystals are widely used in a large number of display devices,such as alphanumeric displays, flat screens, optical valves, etc.

In general, a liquid-crystal-based display device comprises two parallelplates or substrates which ensure that the liquid crystal is confined.These plates or substrates are provided, on the one hand, with means forcontrolling the orientation and the anchoring of the liquid crystal and,on the other hand, electrodes for applying an electric field to theliquid crystal in order to modify the configuration thereof.

In devices based on nematic liquid crystals, which form the subject ofthe present invention, a nematic liquid crystal is used, this beingachiral or chiralized, for example, by adding a chiral dopant, with ahelix pitch of greater than a few micrometers. The orientation and theanchoring of the liquid crystal near the surfaces are defined byalignment layers or treatments applied to the substrates, which, in theabsence of an electric field, impose a uniform or slightly twistedtexture.

Most of the devices proposed and produced hitherto are monostable. Inthe absence of an electric field, only a single texture is produced inthe device. This texture corresponds to an absolute minimum of the totalenergy of the cell. In an electric field, this texture is continuouslydeformed and its optical properties vary depending on the appliedvoltage. On cutting off the field, the nematic returns again to thesingle monostable texture.

Another class of nematic displays is that of bistable nematics. In thiscase, at least two distinct textures may be produced in the cell. Thesetextures correspond to the same surface anchoring states and are stableor metastable in the absence of an electric field. The switching betweenthe two states is carried out by applying appropriate electricalsignals. However, once the image has been written, it remains stored inthe absence of a field thanks to the bistability (or metastability).This memory of bistable displays is very attractive for manyapplications. It allows a low image refresh rate. It thus makes itpossible, for example, to decrease the consumption of portableappliances.

A typical example of a bistable display is shown schematically inFIG. 1. A description of this device will be found in documents [1] and[2]. The two bistable textures T₀ and T₁₈₀, shown in FIGS. 1 a and 1 brespectively, differ from each other by a twist of ±180° and aretopologically incompatible. The spontaneous pitch of the nematic ischosen to be approximately equal to one quarter of the thickness of thecell in order to make the T₀ and T₁₈₀ energies essentially equal.Without a field, no other state with a lower energy exists: T₀ and T₁₈₀exhibit true bistability. Under a strong electric field, an almosthomeotropic texture H (FIG. 1C) is obtained, with at least one of theanchoring points on the substrates (broken): the molecules are normal tothe plate or substrate near its surface. On cutting off the electricfield, the cell is guided towards one or other of the bistable states,favouring elastic coupling (T₀) or hydrodynamic coupling (T₁₈₀) betweenthe two surface anchoring points. Optically, the two states T₀ and T₁₈₀are very different and make it possible to display black and whiteimages with a contrast of greater than 100.

Another class of memory liquid-crystal devices are displays of theorder-disorder type. This devices have a uniform or regular texture, anda large number of disordered textures, with a high defect density. Suchdevices are mentioned in documents [3] and [4]. In these devices, thelight is strongly scattered and de-polarized on the defects. The opticalproperties of the disordered textures change proportionally to thedefect density, thereby allowing grey scales to be displayed, this beingindispensable for obtaining high-quality images in black, grey and whiteor in colour.

However, the bistable devices proper, with only two equal-energy statesof regular textures, are intrinsically poorly suited for a displayhaving grey tones. This is because in these devices only two distinctbistable textures can be displayed and kept in each of the pixels of theimage.

Various approaches have been proposed in order to try to make bistabledevices compatible with a grey-scale display.

For high-speed displays (for example video screens), it is possible toobtain grey scales by digitization of the image and temporal cutting ofthe video frame into several subframes with durations in ratios of1:2:4, etc. In each subframe, the state of the pixel varies in order toensure on average the digital level of desired intensity. This approach,suitable only for ultrafast devices (for example ferroelectricliquid-crystal displays), has many difficulties. It requires complicateddrivers (frame memory requirement). In addition the multiplexing isdifficult (requiring very short subframes to be produced). Finally, thisapproach sacrifices the bistability. This is because the grey scales arenot actual states of the pixel but a temporal average of the bistablestates and, when image refreshing is cut, the image stops. It displaysthe last state of the pixels. This is a black-and-white image.

Another approach consists in spatially subdividing each of thepixels—the digitized image is displayed as a spatial average of theblack-and-white states produced in the sub-pixels. The bistability ofthe image is preserved, but the device is complicated (very smallsub-pixel dimensions, considerable increase in the number of sub-pixelsto be addressed for the same spatial resolution, difficulties inmultiplexing).

The objective of the present invention is to provide novel means forproducing a liquid-crystal-based display suitable for allowing greyscales to be obtained.

This objective is achieved within the context of the present inventionby means of a nematic bistable display device comprising:

-   -   a nematic liquid-crystal layer placed between two substrates        provided with electrodes and with alignment means which ensure        monostable surface anchoring states of the liquid crystal, the        symmetry and the force of at least one of the anchoring states        allowing the anchoring to be broken in an electric field        perpendicular to the substrates;    -   means for applying an electric field to the liquid crystal,        between the two substrates, and for breaking the anchoring to at        least one of the surfaces, with a transient passage of the        surface director parallel to the field and relaxation after the        field has been cut off towards one or other of two bistable or        metastable textures, which differ from each other by a 180°        twist and are both compatible with monostable anchoring states;        characterized in that it furthermore includes:    -   control means for producing, after the anchoring has been        broken, hybrid textures in which the said bistable textures        coexist in a controlled proportion in the same pixel, the said        textures being separated by 180° disclination lines in the        volume or by 180° reorientation walls on one of the surfaces;        and    -   means for long-term stabilization of the hybrid textures by        transformation of the volume lines into surface walls and        immobilization of these walls on the surface.

Within the context of the present invention, the liquid crystal used maybe doped in order to become chiral and may correspond to a cholesteric.

The present invention also relates to a display method employing anematic bistable display device comprising:

-   -   a nematic liquid crystal layer placed between two substrates        provided with electrodes and with alignment means which ensure        monostable surface anchoring states of the liquid crystal, the        symmetry and the force of at least one of the anchoring states        allowing the anchoring to be broken in an electric field        perpendicular to the substrates; and    -   means for applying an electric field to the liquid crystal,        between the two substrates, and for breaking the anchoring on at        least one of the surfaces, with a transient passage of the        surface director parallel to the field and relaxation after the        field has been broken towards one or other of two bistable or        metastable textures, which differ from each other by a 180°        twist and are both compatible with monostable anchoring states;        characterized in that the method comprises the steps consisting        in:    -   producing, after the anchoring has been broken, hybrid textures        in which the said bistable textures coexist in a controlled        proportion within the same pixel, the said textures being        separated by 180° disclination lines in the volume or by 180°        reorientation walls on one of the surfaces; and    -   stabilizing the hybrid textures long term by transformation of        the volume lines into surface walls and immobilization of these        walls on the surface.

Further features, objectives and advantages of the present inventionwill become apparent on reading the detailed description which follows,in conjunction with the appended drawings, these being given by way ofnon-limiting examples and in which:

FIG. 1 shows schematically a bistable liquid-crystal display cellaccording to the prior art, FIGS. 1 a and 1 b illustrating two stablestates of the cell and FIG. 1 c illustrating a transient state in anelectric field;

FIG. 2 shows a photograph of a pixel of a liquid-crystal display cellaccording to the present invention;

FIG. 3 shows the architecture according to the present inventionresulting from a surface defect on a confinement plate, FIG. 3 a showingthe cell in an electric field and FIG. 3 b showing the cell after theelectric field has been cut off;

FIGS. 4 a and 4 b illustrate two types of defect lines capable ofexisting in a display device according to the present invention;

FIG. 5 shows schematically the energy of a volume defect line V as afunction of its distance z from the surface;

FIG. 6 shows the orientation of the director of the nematic, in amoderate electric field, intermediate between the Fredericks thresholdand the break threshold of the anchoring;

FIGS. 7 a and 7 b show schematically the variation from a volumedisclination line to a surface wall;

FIG. 8 shows, for two polarities, the amplitude of a “sticking” voltagecapable of transforming volume disclination lines into surface walls;

FIG. 9 shows the optical transmission of a cell provided with twopolarizers, as a function of the amplitude of fixed-duration switchingpulses; and

FIG. 10 shows six different grey levels obtained on a cell according tothe present invention.

The typical architecture of a bistable nematic pixel, switched bybreaking the anchoring, is shown schematically in FIG. 1. A thin layerof nematic liquid crystal 10, having a thickness d<5 μm, preferably d<2μm, is contained between two substrates 20, 30 (made of glass, plastic,etc.). Such substrates 20, 30 are also called plates. Two electrodes 22,32, at least one of which is transparent, are deposited respectively onthe inner face of the substrates 20, 30, in order to allow an electricfield perpendicular to the substrates 20, 30 to be applied to the liquidcrystal. Alignment layers 24 and 34 define monostable anchoring stateson the substrates 20, 30: in the absence of a field, a singleorientation of the nematic director n is imposed on each surface by thealignment layer 24, 34. This direction is also called the easy axis e.

On the plate 20 (master plate), the alignment is planar or oblique, witha high zenithal anchoring energy: during switching, in a high electricfield, the orientation of the surface director n₁ remains close to theeasy axis e₁ and after the field has been cut off n₁ again becomesparallel to e₁.

On the plate 30 (slave plate), the orientation is planar (easy axis e₂parallel to the plane of the cell) and the zenithal anchoring is weak ormoderate. In a high electric field, the anchoring to this plate breaks(FIG. 1 c): the surface director n₂ orients parallel to the field andperpendicular to e₂. This orientation corresponds to the maximumanchoring energy and to a zero anchoring torque. After the electricfield has been cut off, the director n₂ is in unstable equilibrium(absence of torque) and may return to equilibrium in two different ways:with n₂ parallel (T₀, FIG. 1 a) or antiparallel (T₁₈₀, FIG. 1 b) to e₂.This is because these two states are also compatible with the monostableanchoring state (the nematic is a system of the quadripolar kind and then₂ and −n₂ states are equivalent). In this way, after switching bybreaking the anchoring, one or other of the textures (T₀ or T₁₈₀) isproduced.

The choice of the final texture is made by weak coupling between the twoanchoring states, transmitted by the nematic 10, which disturb theunstable equilibrium of the slave plate 30 upon cutting off the field. Asudden cut-off favours hydrodynamic coupling (volume and surfacebackflow effect) and produces the twisted texture T₁₈₀. A gradualcut-off favours static coupling (by bending elasticity) and imposes thequasi-uniform final state T₀.

The two textures T₀ and T₁₈₀ are topologically distinct: a continuoustransition between them is impossible as this requires breaking eitherthe anchoring or the nematic order. For an achiral nematic, the T₁₈₀texture is metastable. That is to say it has a higher energy than T₀,but a high energy barrier prevents it from spontaneously transforminginto T₀ in the absence of a field. Long-term, T₁₈₀ may neverthelesstransform to T₀ by nucleation and propagation of defects (180°disclination lines). This spurious switching may be prevented bychiralizing the nematic with chiral dopants having a pitch P close toP=4d. In this way the energies of the two textures become equal and truebistability is achieved: a bistable nematic display is obtained whichoperates in black and white or more precisely in “on/off” mode.

The grey scale display device according to the present invention has thesame basic structure illustrated in FIG. 1 and described previously.

This is because, in FIG. 3 et seq., which illustrate cells according tothe present invention, there are again two plates 20, 30 provided withelectrodes 22, 32 and with alignment layers 24 and 34, in accordancewith the arrangements described above. FIG. 3 also shows, schematicallyillustrated, means 40 connected to the electrodes 22, 32, these beingsuitable for applying between the electrodes, and therefore on theliquid-crystal material 10, electrical drive pulses of controlledamplitude and controlled duration.

The inventors have in fact demonstrated that in this type of device, itis possible to produce and stabilize controllable intermediate analoguegrey states in order to obtain a bistable display with grey tones. Thesestates correspond to “hybrid” textures, that is to say the two bistabletextures coexist in the same pixel (FIG. 2). The hybrid state istherefore a random mixture of two types of domain—“written” domains(black texture T₁₈₀ between crossed polarizers) and “erased” domains(diagonally oriented white texture T₀ between crossed polarizers).Locally, a single state is produced in each of the domains, which isuniform over the entire area of the domain. The area of the domains isof the order of 100×d², small compared with the total area of the pixel,and a large number of microdomains are produced within the same pixel,making it possible to produce a large number of grey levels by varyingtheir average density. The transmitted or reflected intensity of thepixel is therefore a weighted average of the intensities of the twotypes of domain and may vary continuously between 0% (black state) and100% (white state).

The origin of the hybrid states is the unstable equilibrium of thesurface director after the anchoring at the slave plate 30 has beenbroken. In this state, the slightest disturbance is sufficient to inducea rapid relaxation of the nematic surface director n₂ towards one orother of the two positions parallel or antiparallel to e₂ (FIG. 1) andtherefore towards the final texture T₀ or T₁₈₀. If the voltage of thedrive pulse greatly exceeds the threshold for breaking the anchoring,the static and dynamic coupling between the two surfaces are strong andthe entire pixel will switch towards a single uniform texture (whitestate or black state). On the other hand, close to the breakingthreshold, the coupling is weak and local perturbations (these may beconsidered as local “noise” which is superimposed on the drive signal)are sufficient to induce the hybrid states.

The inventors have determined that local perturbations may be created bycontrolling the dispersion in the surface anchoring force. This isbecause the local surface anchoring energy may vary within certainlimits which depend on the nature and the uniformity of the alignmentlayers. In this way, a uniform drive field may or may not locally exceedthe anchoring breaking threshold. It will induce hybrid final textures,the grey level of which depends on the voltage of the pulses.

The inventors have also demonstrated that the local perturbations may becreated by the topography of the surface, for example a variablethickness of the alignment layer. This topography may be periodic(one-dimensional or two-dimensional surface grating) or random, forexample a microstructured surface. In these cases, the voltage forbreaking the anchoring varies with the local thickness of theliquid-crystal film since the field is no longer uniform. It is screenedto a greater or lesser extent by the dielectric properties of theanchoring layer.

The inventors have also determined that hybrid textures may be obtainedby applying slightly non-uniform electric fields to a pixel using, forexample, electrodes the resistance or surface finish of which is notuniform. This gives a field whose surface intensity or whose orientationvaries (slightly oblique field). This disturbs to a greater or lesserextent the breaking of the anchoring.

FIG. 3 shows schematically the inhomogeneous electric field createdaround an irregularity 36 in the surface of the electrode 32 on theslave plate 30 and the induced perturbations in the texture in a field(FIG. 3 a). After relaxation on cutting off the field (FIG. 3 b), ahybrid texture is obtained, with the two bistable textures induced oneach side of the surface irregularity.

One means according to the present invention of producing the hybridtextures is to cause, during the drive pulse, transient textureinstabilities which are hydrodynamic or flexoelectric in origin or aredue to the intrinsic polarity of the surface. All these instabilities,of dynamic or static origin, lead to periodic deviations in the nematicdirector with respect to its equilibrium state in the volume and at thesurface, with a period comparable to the thickness of the cell. In astrong field, the amplitude of these instabilities is very small butwhen the field is close to the breaking threshold they may be sufficientto induce hybrid textures.

Another means, according to the present invention, allowing hybridtextures to be produced is associated with the transient shear flows ofthe liquid crystal during application of the field (backflow effect) onthe surface and in the volume, which flows also disturb the orientationof the director. On inhomogeneous or rough surfaces, these flows causeslight transient inhomogeneities in the orientation of the surfacedirector. Cut-off of the field after the anchoring has been broken, butbefore complete relaxation of the flows, therefore also induces onceagain a hybrid pixel texture.

However, the inventors have found that, once the hybrid textures havebeen created, they may vary over time and relax towards black or whiteuniform states by propagation of the lines of defects which separate thedomains corresponding to the two main bistable textures.

More specifically, they have found that, in the geometry of the displaywhich forms the subject of the present invention, two types of lines ofdefects may exist, with different structures and mobilities.

A first type of line of defects is illustrated in FIG. 4 a. These are180° disclination lines. These defect lines (shown schematically in theform of the region V in FIG. 4 a) have a molten, isotropic or biaxialcore. At this point the nematic order parameter becomes zero or evenchanges sign. In equilibrium, these lines lie in the mid-plane of thecell and form closed loops which surround the domains making up thehybrid texture. These lines are highly mobile and can move with viscousrubbing in the plane of the device owing to the action of the elasticforces or of the flows. If the distortion energy per unit area of twobistable textures is different (for example achiral or cholestericnematic with a pitch poorly matched to the thickness of the cell), anelastic force acts on the line and displaces it. Locally, after the linehas passed, the higher-energy texture (metastable texture) is replacedwith the other texture. The area occupied by the metastable texturegradually decreases and long-term the entire pixel becomes uniform inthe stable state (white or black). Even for bistable textures with thesame energy (in the case of a cholesteric well matched to the thicknessof the cell), the tension in the defect line (energy per unit length)makes the disclination loops shrink and disappear long term, causing aslow change in the grey scale towards 0% or 100%.

Another possible structure of the lines of defects is shownschematically in FIG. 4 b. In this case, they are 180° surface directorreorientation walls (the region S in FIG. 4 b) which separate theregions in which the two bistable textures exist. Across the wall, thesurface director gradually rotates through 180°. These lines, localizedon the surface, are much less mobile than the volume lines because oftheir higher effective viscosity and, above all, a “strong” friction dueto the attachment of the line to the surface irregularities and/or tothe local memory of the anchoring. Due to the strong friction, there isa threshold for the force applied to the line, below which the lineremains “stuck” to the surface and does not move. For most surfaces,this threshold is very high and even in the case of achiral nematics theenergy difference between the two textures is insufficient to exceedthis threshold. Thanks to the lines of defects “sticking”, an infinitebistability is obtained for arbitrary grey scale states.

The energy per unit length of the two lines of defects is very different(FIG. 5). The surface walls have a lower total volume distortion energyand a zero core energy (they do not have a molten core and in the wallthe nematic order is not greatly disturbed). In contrast, they have astrong anchoring energy contribution due to the disorientation of thesurface director in the wall with respect to the easy axis imposed bythe alignment layer. In general, the total energy is lower in the caseof surface walls, thereby favouring the transformation of thedisclination lines into the surface walls. For this transformation totake place, it is necessary to bring the core of the line close to thesurface (the core “passes through” the surface and becomes a virtualcore). FIG. 5 shows schematically the energy of the volume line V as afunction of its distance z from the surface. A high anchoring energybarrier (B) separates the S state (surface wall) from the V state(volume line) and prevents the disclination line from comingsufficiently close to the surface. Because of this barrier, in theabsence of an electric field the volume lines V remain in the middle ofthe cell, without being transformed into surface walls.

After the anchoring has been broken, when a hybrid texture forms, thetwo types of structure of the defect lines are present, sometimes evenin different regions of the same loop. The highly mobile volume linestherefore move under the action of the line tension force and possiblythe force due to a difference in the energies of the two bistabletextures (if the chiral pitch deviates from its optimum value). Thepixel will therefore relax long term towards another grey scale,different from that imposed by the drive pulse, as only the domainscompletely surrounded by surface lines will survive.

To obtain grey scales which are stable long term, it is thereforenecessary to “stick” all the disclination lines to the surface bytransforming them into 180° reorientation walls. The inventors havediscovered that this transformation can be effectively achieved byapplying electrical pulses of appropriate voltage and duration. Under amoderate electric field, intermediate between the Fredericks thresholdand the anchoring breaking threshold, the director of the nematicorients in the volume parallel to the field, except in the regions closeto the two surfaces (FIG. 6). The two main bistable textures transformin a field in different ways: T₀ tilts everywhere in the same direction,whereas the “half-turn” texture in torsion T₁₈₀ retains its topological180° rotation constraint, becoming a “half-turn” in bending. Close tothe master plate 20, with strong oblique anchoring, the distortion ofthe director is uniform. On the slave plate 30, with weak planaranchoring, the surface director is slightly oblique because of theelectric torque. However, the two textures have opposite signs (±α). A2α surface reorientation wall is therefore created opposite thedisclination. Between the surface wall and the volume line, the directorremains perpendicular to the electric field under the topologicalconstraint and a large amount of electrical energy is stored in thisregion. To minimize this energy, the line is pushed by the correspondingelectric force towards the surface (FIG. 7 a). If the electric force ishigh enough to exceed the energy barrier, the disclination linetransforms into a surface wall (FIG. 7 b) with a virtual core.

The threshold voltage U_(s) for the defects to stick, that is to say forthe volume disclination lines to be transformed into surface walls,depends on the thickness of the cell, the surface anchoring force, thepulse polarity and the pulse duration τ (because of the friction in thecase of the line being transported towards the surface). The typicalvalue of Us is a few volts.

FIG. 8 shows the U_(c)(τ) dependence, measured experimentally for a cellaccording to the present invention of the type described above. Thisshows that relatively small and short “sticking” signals allow thedefect lines to be stabilized and infinite bistability of the hybridtextures to be obtained. These signals may be sent independently of theswitching signals, after the relaxation of the broken anchoring, or theymay be incorporated into the last part of the drive signals.

ILLUSTRATIVE EXAMPLE

The example below corresponds to a non-limiting example of the deviceproposed in the present invention, produced and studied by theinventors.

A liquid-crystal cell 10 having a thickness of 1.6 μm was mountedbetween two glass plates 20, 30 having a thickness of 1.1 mm. Theseplates 20, 30 were coated with an electrically conductive transparentlayer of a mixed indium tin oxide. The master plate 20 received anevaporation of silicon monoxide at an 85° grazing angle. The anchoringof the liquid crystal 10 was thus strong and tilted at about 30° to thismaster plate 20. The slave plate 30 received an evaporation at a 75°angle of inclination. The anchoring obtained on this slave plate 30 wasweak planar anchoring. The liquid crystal used was pentylcyanobiphenyl(5CB) doped with material S811 (from Merck) in order to obtain a 90°spontaneous twist over the thickness of the cell at the temperature ofthe laboratory. The two states of the cell thus have the same energy.

The inventors have observed, with the aid of a microscope fitted with aconventional image recording device, the behaviour of the domains afterswitching pulses close to the threshold have been applied.

The curve in FIG. 8 gives the values of “sticking” pulses of the twopolarities which allow the domains in this cell to be set. Thedifference between the two polarities is explained when the local fieldproduced by the surface is taken into account.

The curve in FIG. 9 shows the optical transmission of the cell providedwith two polarizers as a function of the amplitude of switching pulsesof fixed duration (800 μs). This FIG. 9 shows a white state, for aswitching pulse amplitude of less than 10 V, grey states which graduallychange from white to black, for switching pulse amplitudes varyingbetween 10 and 14 V, and a black state for switching voltages greaterthan 14 V. The infinite stability of the grey scales (resulting from theapplication of switching pulses of amplitude between 10 and 14 V) wasobtained by applying, after these switching pulses, a sticking voltageof 5 volts and of the same duration (800 μs). More generally, thisswitching voltage is between 2 V and 10 V.

FIG. 10 shows six different grey scales produced in this cell (states L2to L7 in FIG. 9).

In short, the present invention provides a grey-scale bistable nematicdisplay device. The device uses a known display principle in its generalstructure, employing two twisted bistable textures of 0° and 180°respectively. The switching is obtained by breaking the anchoring underthe action of drive pulses. With signals close to the breakingthreshold, a hybrid state is obtained in each pixel, the two bistabletextures being respectively produced in microdomains. The ratio of theareas occupied by the two states (and therefore the corresponding greyscale) is controlled by the drive voltage). The grey state of the pixelis maintained without any long-term change by a controlledtransformation of the volume disclination lines between the microdomainsinto surface walls (which are stable to infinity because of the strongrubbing on the surface).

Of course, the present invention is not limited to the particularembodiment that has just been described but extends to all variants inaccordance with its spirit.

BIBLIOGRAPHY

-   [1] “Fast bistable nematic display using monostable surface    switching”, I. Dozov, M. Nobili, G. Durand, Appl. Phys. Lett. 70,    1179 (1997).-   [2] WO-A-97 17632.-   [3] “Bistable display device based on nematic liquid crystals    allowing grey tones”, R. Barberi, G. Durand, R. Bartolino, M.    Giocondo, I. Dozov, J Li, EP 0773468, U.S. Pat. No. 5,995,173, JP    9274205 (1998).-   [4] D. K. Yang, J. L. West, L. C. Chien and J. W. Doane J. Appl.    Phys. 76, 1331 (1994).

1. A nematic bistable display device comprising: a nematicliquid-crystal layer (10) placed between two plates(20, 30) providedwith electrodes (22, 32) and with anchoring means (24, 34) which ensuremonostable surface anchoring states of the liquid crystal (10) definingtwo stable textures without electrical field, one texture having theliquid crystal molecules mainly parallel and the other texture having atwist about +/−180° from one plate to the other, along a directionnormal to the plates, the symmetry and the force of at least one of theanchoring states allowing the anchoring to be broken in an electricfield perpendicular to the plates(20, 30); means (40) for applying anelectric field to the liquid crystal, between the two plates (20, 30),and for breaking the anchoring to at least one of the surfaces, with atransient passage of the surface director parallel to the field andrelaxation after the field has been cut off towards one or other of saidtwo stable textures, which differ from each other by a 180° twist fromone plate to the other, along a direction normal to the plates, and areboth compatible with monostable anchoring states; wherein it furthermoreincludes: control means (40) for producing, after the anchoring has beenbroken, hybrid textures in which the said two stable textures coexist ina controlled proportion in a pixel, one texture corresponding to abright state, the other corresponding to a dark state, a grey statevalue being proportional to the density in each bright and dark states,the said textures being separated by 180° disclination lines in thevolume or by 180° reorientation walls on one of the surfaces; and means(40) for then applying on the liciuid-crystal layer electric pulses tostick all the 180° disclination lines to the surface by transforming the180° disclination lines into 180° reorientation walls, for long-termstabilization of the hybrid textures by immobilization of these 180°reorientation walls on the surface.
 2. The device according to claim 1,wherein the liquid crystal (10) is doped so as to be chiral and tocorrespond to a cholesteric.
 3. The device according to claim 1 or claim2, wherein the control means (40) are designed to apply to the liquidcrystal an electric field close to the threshold for breaking theanchoring of the liquid crystal to at least one of the surfaces.
 4. Thedevice according to claim 1, wherein the control means (40) comprise atleast one alignment layer, the surface of which is periodically orrandomly corrugated.
 5. The device according to claim 1, wherein thecontrol means (40) comprise means capable of defining a dispersion inthe anchoring force on at least one of the surfaces.
 6. The deviceaccording to claim 5, wherein the control means (40) comprisenon-uniform alignment layers.
 7. The device according to claim 5,wherein the control means (40) are suitable for applying a uniformelectric drive field.
 8. The device according to claim 1, wherein thecontrol means comprise means (40) capable of applying slightlynon-uniform electric fields to a pixel.
 9. The device according to claim8, wherein the control means (40) comprise electrodes (22, 32) theresistance or the surface finish of which is not uniform.
 10. The deviceaccording to claim 1, wherein the control means comprise means (40)capable of causing, during a drive pulse, transient textureinstabilities which are of hydrodynamic or flexoelectric origin, or aredue to the intrinsic polarity of the surface.
 11. The device accordingto claim 1, wherein the control means comprise means (40) capable ofcausing, during a drive pulse, transient shear flows of the liquidcrystal.
 12. The device according to claim 1, wherein the stabilizingmeans (40) comprise means capable of applying electrical pulses to theliquid crystal, the amplitude of which is between the Fredericksthreshold and the anchoring breaking threshold.
 13. The device accordingto claim 1, wherein the stabilizing means (40) comprise means capable ofapplying electrical pulses to the liquid crystal with an amplitude of afew volts.
 14. The device according to claim 1, wherein the stabilizingmeans (40) comprise means capable of applying electrical pulses to theliquid crystal with an amplitude of between 2 V and 10 V.
 15. The deviceaccording to claim 1, wherein the stabilizing means (40) comprise meanscapable of applying electrical pulses to the liquid crystal,independently of switching signals which ensure that the anchoring isbroken, after relaxation of the broken anchoring.
 16. The deviceaccording to claim 1, wherein the stabilizing means (40) comprise meanscapable of applying electrical pulses to the liquid crystal which areincorporated into the last part of drive signals for ensuring that theanchoring is broken.
 17. The device according to claim 1, wherein thecontrol means (40) are designed to apply to the liquid crystal avariable electric field close to the threshold for breaking theliquid-crystal anchoring on at least one of the surfaces, the amplitudeof the field being used to control the grey scale obtained.
 18. A methodemploying a nematic bistable display device comprising: a nematic liquidcrystal layer (10) placed between two plates (20, 30) provided withelectrodes (22, 32) and with alignment means (24, 34) which ensuremonostable surface anchoring states of the liquid crystal defining twostable textures without electrical field, one texture having the liquidcrystal molecules mainly parallel and the other texture having a twistof about +/−180° from one plate to the other, along a direction normalto the plates, the symmetry and the force of at least one of theanchoring states allowing the anchoring to be broken in an electricfield perpendicular to the plates; and means (40) for applying anelectric field to the liquid crystal, between the two plates, and forbreaking the anchoring on at least one of the surfaces, with a transientpassage of the surface director parallel to the field and relaxationafter the field has been broken towards one or other of said two stabletextures, which differ from each other by a 180° twist from one plate tothe other, along a direction normal to the plates, and are bothcompatible with monostable anchoring states; wherein the methodcomprises the steps of: producing, after the anchoring has been broken,hybrid textures in which the said two stable textures coexist in acontrolled proportion within a pixel, one texture corresponding to abright state, the other corresponding to a dark state, a grey statevalue being proportional to the density in each bright and dark states,the said textures being separated by 180° disclination lines in thevolume or by 180° reorientation walls on one of the surfaces; and thenapplying on the liquid-crystal layer electric pulses to stick all the180° disclination lines to the surface by transforming the 180°disclination lines into 180° reorientation walls, for long-termstabilization of the hybrid textures by immobilization of these 180°reorientation walls on the surface.
 19. The method of claim 18, whereinthe step of applying stabilizing electric pulses comprises applicationof electrical pulses the amplitude of which is between the Fredericksthreshold and the anchoring breaking threshold.
 20. The method of claim18, wherein the step of applying stabilizing electric pulses comprisesapplication of electrical pulses having an amplitude of a few volts. 21.The method of claim 18, wherein the step of applying stabilizingelectric pulses comprises application of electrical pulses having anamplitude between 2 and 10 volts.
 22. The method of claim 18, whereinthe alignment means comprises local variation of thickness defining nonuniform thickness of the liquid-crystal layer.